Name boolean arguments

What would you prefer?

    adaptToMemberWithArgs(tree, qual, name, mode, false, false)

Or:

    adaptToMemberWithArgs(tree, qual, name, mode, reportAmbiguous = false, saveErrors = false)

compiler, reflect, and library have been swept.

A sanity check is included to show that using named args for clarity incurs no bytecode penalty.

Review by @paulp. Minor chuckles here: https://github.com/retronym/scala/pull/new/topic/bool-arg#L62L15